A nitrile group-containing highly saturated, copolymer rubber typified by a hydrogenated acrylonitrile-butadiene copolymer robber is excellent in heat resistance, oil resistance, ozone resistance and the like as compared with a common nitrile group-containing copolymer rubber such as an acrylonitrile-butadiene copolymer rubber, having a plenty of carbon-carbon unsaturated bonds in the main chain structure thereof.
As such a nitrile group-containing highly saturated copolymer rubber. Patent Document 1 has proposed a nitrile group-containing highly saturated copolymer rubber, comprising unsaturated nitrile monomer units, butadiene monomer units and isoprene monomer units, with a molar ratio between the butadiene monomer unit and the isoprene monomer unit of 3:1 or less. The technique described in Patent Document 1 seeks to improve the compression set resistance and to improve the flexibility of the nitrile group-containing highly saturated copolymer rubber at low temperatures, in addition to the above-mentioned characteristics possessed by the nitrile group-containing highly saturated copolymer rubber. Further, the nitrile group-containing highly saturated copolymer robber of Patent Document 1 is suitable for the resistance to hardening in oil. However, the cross-linked, rubber obtained by using the nitrile group-containing highly saturated copolymer rubber described in Patent Document 1 is fundamentally low in resistance to swelling in oil so as to be insufficient for practical use.